Bucket loaders have been known and used for many years. The customary non-articulated bucket addresses a pile of material from the front. The inertia and resistance of a large or heavy pile enables the front edge of the bucket to undercut a portion of the material provided the bucket is advanced with sufficient speed. Then, when the bucket is tilted upwardly the bucket is filled. With a small or light pile of material, on the other hand, the resistance of the pile to the force of the entering, undercutting blade is insufficient, and the small pile retreats from the advancing blade. A backstop of some kind is therefore needed when a conventional bucket is used to pick up small piles. The backstop often consists of a broom held firmly against the advancing bucket and pile. Labor cost is thereby incurred.
Articulated buckets with opposing-action bucket halves are also well known. The clamshell bucket is a common example.
In the classical clamshell, the material-entering edge of each bucket half is parallel to the pivotal axis as the edge sweeps through an arcuate path. This is exemplified in the devices of R. T. MacAlpine et al U.S. Pat. No. 2,949,201 and P. E. La Tendresse U.S. Pat. No. 2,788,143. Owing to the fact that the entering edges of the bucket halves in these patents sweep in an arcuate manner, with foward components of motion, it is necessary that the material being loaded be restrained in some manner from moving away from the advancing cutting edges.
Where, as in Mac Alpine, the material is located in situ on the face of a mine shaft, there is no problem since the mass is stationary. In the usual case, however, the mass is not so located and some kind of restraint is necessary. This is why the axis of a clamshell bucket is often horizontal and elevated above the material being addressed, the underlying ground serving to prevent the material from moving away from the bucket halves, as in the La Tendresse disclosure.